1. Field of the Invention
The present invention relates to a connection sheet for firmly bonding an electronic component such as a semiconductor chip to a circuit board to achieve electrical connection between electrodes of the two components, and also to electrode connection structure and method using the connection sheet.
2. Description of the Related Art
With a recent trend to smaller-sized, thinner electronic components, electrical circuits used in such components have come to be increased in density and reduced in connection pitches. Since connection of an electronic component to fine electrodes is difficult to achieve with conventional techniques utilizing soldering, rubber connectors or the like, in recent years anisotropic conductive adhesives and film-like materials (connection sheets) which can provide excellent resolution are widely used.
A connection sheet comprises an adhesive having a predetermined content of an electrically conductive material, such as conducting particles. With the connection sheet interposed between an electronic component and electrodes or an electrical circuit, pressure or both heat and pressure are applied to the connection sheet, whereby the two components are firmly bonded together so that corresponding electrodes of the two components can be electrically connected to each other while providing insulation between adjacent electrodes.
The basic idea for achieving high resolution with the connection sheet is that the particle diameter of conducting particles in the vicinity of the electrodes shall be smaller than the length of an insulating region between adjacent electrodes, so as to ensure inter-electrode insulation performance. Also, the content of conducting particles in the connection sheet is selected to obtain a density such that the particles do not come into contact with one another, and that when electrodes are connected, the particles never fail to exist on the electrodes to be connected, to thereby achieve conductivity at connecting portions.
However, if the particle diameter of the conducting particles is too small, the conducting particles undergo cohesion and are united due to an excessive increase in the surface area of the particles, making it impossible to maintain required insulation performance between adjacent electrodes. On the other hand, if the content of the conducting particles is reduced, the number of conducting particles on the electrodes to be connected also decreases, with the result that conduction between corresponding electrodes cannot be achieved due to deficiency in the number of contact points. With conventional techniques, therefore, it is very difficult to achieve high resolution with a connection sheet while maintaining long-term connection reliability. Specifically, there has recently been an increasing demand for higher resolution, that is, for reduction in the area of electrodes and in the spacing between adjacent electrodes. When pressure or both heat and pressure are applied to the connection sheet in the connection step, however, the conducting particles on the electrodes flow together with the adhesive to a region between adjacent electrodes, thus impeding achievement of high resolution with the use of a connection sheet. If the viscosity of the adhesive is increased in order to suppress such outflow, then the conducting particles do not satisfactorily contact with each electrode, making it impossible to achieve electrical connection between electrodes facing each other. On the other hand, if the viscosity of the adhesive is decreased, not only the conducting particles are more likely to flow but also air bubbles are liable to be contained in the region between adjacent electrodes, lowering the connection reliability, especially, the moisture resistance.
In view of the drawbacks, Unexamined Japanese Patent Publications (KOKAI) No. 61-195179and No. 4-366630, for example, disclose a multi-layer connection sheet including an insulating adhesive layer (first adhesive layer) and a layer (second adhesive layer) filled with conducting particles and separate from the first adhesive layer, wherein the viscosity of the second adhesive layer is selected such that the second adhesive layer exhibits relatively higher viscosity or cohesive force than the first adhesive layer at the time of connection, to lessen the flow of the conducting particles and thereby trap conducting particles on the electrodes.
According to the disclosed techniques, however, the layer filled with conducting particles has higher viscosity than the insulating adhesive layer at the time of connection; therefore, the conducting particles insufficiently contact with the electrodes, increasing the connection resistance and lowering the connection reliability. In order to reduce the connection resistance, a structure may be employed wherein the conducting particles are exposed in advance to the surfaces of the particle-filled layer so that they may be easily brought into contact with the electrodes. In this case, however, the particle diameter of the conducting particles must be increased, which makes it difficult to achieve high resolution.
There has also been proposed a connection sheet which permits connection to fine electrodes or a circuit and which provides excellent connection reliability, wherein conducting particles are concentrated at regions where electrodes are to be connected. Although this connection sheet permits connection to dot-like fine electrodes such as electrodes on a semiconductor chip, it is necessary that the particle-concentrated regions should be accurately aligned with corresponding dot-like electrodes, lowering the operation efficiency.
The present invention was created to solve the above problems, and an object thereof is to provide a connection sheet of which conducting particles can be reliably trapped between electrodes facing each other at the time of connection and can be easily brought into contact with the electrodes, and which is excellent in long-term connection reliability and in operation efficiency during electrode connection work.
According to a first aspect of the present invention, there is provided a connection sheet interposed between electrodes facing each other to join the electrodes together for electrically connecting the electrodes to each other, which comprises: a first adhesive layer made of a first adhesive having an electrical insulating property; and a second adhesive layer placed over the first adhesive layer, the second adhesive layer containing a second adhesive having an electrical insulating property and an electrically conductive material, the second adhesive having a viscosity equal to or lower than that of the first adhesive when the first and second adhesives are in a molten state.
According to the first aspect of the invention, since the viscosity of the second adhesive in a molten state is equal to or lower than that of the first adhesive, when the electrodes are connected the conductive material contained in the second adhesive layer is embedded or trapped in the first adhesive layer having relatively high melting viscosity. Thus, the conductive material can be reliably held between electrodes facing each other. As the first adhesive layer softens and flows thereafter, the conductive material comes into contact with protruding electrodes, permitting electrical conduction. The first adhesive layer has a viscosity equal to or higher than that of the second adhesive and thus can retain the conductive material, whereby no air bubbles are contained in the region between adjacent protruding electrodes.
If the viscosity of the second adhesive containing conducting particles is higher than that of the first adhesive (the difference by which the second adhesive is lower in viscosity than the first adhesive takes a negative value), the viscosity of the second adhesive is so high that the conducting particles cannot be embedded in or trapped by the first adhesive layer. Thus, the conducting particles insufficiently come into contact with the electrodes, making it impossible to achieve electrical connection between electrodes facing each other.
Preferably, when the first and second adhesives are in a molten state, the viscosity of the second adhesive is lower than that of the first adhesive by 1000 poises or less.
If the viscosity of the second adhesive is lower than that of the first adhesive by more than 1000 poises, the viscosity of the second adhesive is so low that outflow of the conducting particles occurs. Also, air bubbles are liable to be contained in the region between adjacent electrodes, lowering the connection reliability, especially the moisture resistance.
The second adhesive layer preferably has a melting viscosity of 500 poises or less.
According to the results of experiments conducted by the inventors, where the melting viscosity of the second adhesive layer itself was 500 poises or less taking account of the viscosity difference with respect to the first adhesive layer, satisfactory connection could be achieved.
The first and second adhesives preferably contain an identical material, since the adhesive strength can be enhanced due to an increase in the adhesiveness at the interface between the first and second adhesive layers.
Preferably, the first adhesive and the second adhesive have different adhesive properties.
The reason is that the connection sheet can be detached preferentially from a substrate surface at an interface with a lower adhesive strength, thus facilitating repair work.
The first adhesive layer and/or the second adhesive layer preferably contains insulating particles.
In the case where the insulating particles are contained, insulation between conducting particles or between a conducting particle and an electrode can be achieved with reliability when the electrodes are connected.
Preferably, the conductive material comprises conducting particles or conducting particles having surfaces thereof coated with an insulating material.
Where the conducting particles are coated with an insulating material, the insulating coating of only those conducting particles located at the electrode contact portion is melted, while the other portion is improved in insulation performance due to the insulating coating.
The first adhesive layer preferably includes a separator sheet which covers a surface of the first adhesive layer opposite the second adhesive layer and which can be peeled off from the first adhesive layer.
In this case, the separator sheet is peeled off when electrodes are actually connected from the first adhesive layer that is, when the connection sheet is actually used, whereby dust or the like is prevented from adhering to the connection sheet before use.
According to a second aspect of the present invention, there is provided a connection structure for joining together electrodes facing each other to electrically connect the electrodes to each other, which comprises: a first adhesive layer made of a first adhesive having an electrical insulating property; a second adhesive layer placed over the first adhesive layer, the second adhesive layer containing a second adhesive having an electrical insulating property and an electrically conductive material the second adhesive having a viscosity equal to or lower than that of the first adhesive when the first and second adhesives are in a molten state; and a substrate disposed for contact with the first adhesive layer or the second adhesive layer, the substrate being provided with an electrode having an electrode surface to be brought into contact with the electrically conductive material, a ratio (L/D) of a longer side (L) to a shorter side (D) of the electrode surface being 20 or less.
According to the second aspect of the invention, in the connection structure for connecting electrodes formed on a substrate such as a semiconductor chip, the ratio (L/D) of the longer side to shorter side of the connection surface of each electrode on the substrate is 20 or less, in which case more conductive material can be reliably trapped on fine protruding electrodes, improving the connection reliability and permitting efficient use of expensive conductive material.
By making the viscosity of the second adhesive in a molten state equal to or lower than that of the first adhesive, as mentioned with reference to the first aspect of the invention, when the electrodes are connected, the conductive material contained in the second adhesive layer is embedded in the first adhesive layer having relatively high melting viscosity, or part of the conductive material is trapped on the electrodes to be connected in contact therewith. Thus, the conductive material can be reliably held between electrodes facing each other.
Preferably, when the first and second adhesives are in a molten state, the viscosity of the second adhesive is lower by 1000 poises or less than that of the first adhesive, as in the first aspect of the invention.
According to a third aspect of the present invention, there is provided a connection structure for joining together electrodes facing each other to electrically connect the electrodes to each other, which comprises: a first adhesive layer made of a first adhesive having an electrical insulating property; a second adhesive layer placed over the first adhesive layer, the second adhesive layer containing a second adhesive having an electrical insulating property and an electrically conductive material; and a pair of electrode rows which face each other and between which the first and second adhesive layers are interposed, at least one of the pair of electrode rows including protruding electrodes protruding from a substrate, the protruding electrodes each having a base near the substrate and a top face facing a corresponding electrode, the first adhesive layer surrounding at least the base of each of the protruding electrodes.
According to the third aspect of the invention, the first adhesive layer as an insulating adhesive layer is placed close to the electrodes protruding from the substrate, thus improving the insulation performance between adjacent electrodes as well as the resolution.
Preferably, when the first and second adhesives are in a molten state, the viscosity of the second adhesive is lower by 1000 poises or less than that of the first adhesive.
In the case where the first adhesive layer as an insulating adhesive layer has high melting viscosity, it is more unlikely that the connection pressure acts upon the region between adjacent electrodes and thus that the conductive material flows to this region, further improving the insulation performance between adjacent electrodes and the resolution.
Preferably, the conductive material of the second adhesive layer has a density such that the density gradually decreases with distance from the top face of each protruding electrode toward the base near the substrate.
The higher the density of the conductive material at the top face or connection surface of the electrode, the more reliable the electrical connection is achieved. For the base side of the electrode, on the other hand, the lower the density of the conductive material, the more reliable the adjacent electrodes are insulated from each other.
According to a fourth aspect of the present invention, there is provided a connection method for joining together electrodes facing each other to electrically connect the electrodes to each other, which comprises: the arranging step of interposing a connection sheet including a first adhesive layer and a second adhesive layer between a pair of electrode rows such that the pair of electrode rows face each other, the first adhesive layer being made of a first adhesive having an electrical insulating property and a thermosetting property, the second adhesive layer being placed over the first adhesive layer and containing an electrically conductive material and a second adhesive having an electrical insulating property and a thermosetting property; and the heat-pressure application step of heating the first and second adhesive layers under pressure, the heat and/or pressure applied to the second adhesive during the heat-pressure application step being lower than the heat and/or pressure applied to the first adhesive.
According to the fourth aspect of the invention, the heat or/and pressure applied to the second adhesive containing the conducting particles is lowered, whereby at the time of connection, the viscosity of the second adhesive having a thermosetting property can be made to be equal to or lower than that of the first adhesive also having a thermosetting property.
Therefore, the viscosity of the second adhesive in a molten state at the time of connection can be made to be smaller than that of the first adhesive layer, as mentioned above with reference to the first aspect of the invention, whereby at the time of electrode connection, the conductive material in the second adhesive layer is embedded in the first adhesive layer having relatively high melting viscosity, or part of the conductive material is trapped on the electrodes to be connected in contact therewith. Thus, the conductive material can be reliably held between the electrodes facing each other. Further, since the viscosity of the first adhesive layer is high relative to that of the second adhesive, not only the conductive material 3 can be trapped but also no air bubbles are contained in the region between adjacent protruding electrodes.
Preferably, in the heat-pressure application step, the first and second adhesive layers are heated under pressure such that the second adhesive is lower in viscosity by 1000 poises or less than the first adhesive when the first and second adhesives are in a molten state.
Also, in the heat-pressure application step, the connection sheet is preferably heated under pressure with a heat source arranged close to the first adhesive layer.
In the case where the heat source is arranged close to the first adhesive layer and a thermosetting resin is used for each of the adhesive layers, the viscosity of the second adhesive which is remote from the heat source becomes lower than that of the first adhesive.
According to a fifth aspect of the present invention, there is provided a connection method for joining together electrodes facing each other to electrically connect the electrodes to each other, which comprises: the arranging step of interposing a connection sheet including a first adhesive layer and a second adhesive layer between a pair of electrode rows such that the pair of electrode rows face each other, the first adhesive layer being made of a first adhesive having an electrical insulating property and a thermosetting property, the second adhesive layer being placed over the first adhesive layer and containing an electrically conductive material and a second adhesive having an electrical insulating property and a thermosetting property; the first heat-pressure application step of heating the second and first adhesive layers under pressure such that the electrically conductive material comes into contact with each pair of electrodes facing each other while the first and second adhesives are in a molten state, thereby permitting electrical connection of each electrode pair; the current supply inspection step of inspecting electrical conductivity of each electrode pair; and the second heat-pressure application step of again heating the first and second adhesive layers under pressure after the current supply inspection step to harden the adhesives.
According to the fifth aspect of the invention, the heat-pressure application step is divided into two stages or more so that the current supply inspection step and/or a repair step for connected electrodes may be carried out as needed between the stages, whereby the operation efficiency and the product quality can be improved.
Preferably, the current supply inspection step is performed while the cohesive force of the first adhesive and/or the second adhesive is increased to such an extent that connection of the electrodes can be maintained.
In this case, with the connection structure maintained as it is, the current supply inspection can be carried out with ease and reliability.
The cohesive force can be increased by partially accelerating the hardening reaction of the adhesives by heating, or by making the temperature of the connecting portion lower than the connection temperature to thereby increase the modulus of elasticity, for example.
The current supply inspection step is preferably performed while pairs of electrodes are kept under pressure so that connection of the electrodes can be maintained to allow electrical connection.
Also in this case, the current supply inspection can be easily and reliably carried out.
Preferably, in the first heat-pressure application step, the first and second adhesive layers are heated under pressure such that the second adhesive has a viscosity equal to or lower than that of the first adhesive when the first and second adhesives are in a molten state.
By making the viscosity of the second adhesive in a molten state equal to or lower than that of the first adhesive, as mentioned above with reference to the first aspect of the invention, at the time of electrode connection, the conductive material in the second adhesive layer is embedded in the first adhesive layer having relatively high melting viscosity, or part of the conductive material is trapped on the electrodes to be connected in contact therewith. Thus, the conductive material can be reliably retained on the electrodes to be connected.